I've got a pair of Keen hiking boots that, at their best when brand new, never had a particularly good grip, and now, a few years later, have simply lost any grip to speak of. They are so slippery on any surface, wet or dry, that not only is it embarrassing, but downright dangerous sometimes.
My question is whether there is some technique or trick to getting the soles grippier? Perhaps scarring them with a blade, or running a grate over them, or some other method? The bottoms appear to be the standard rubbery type of bottom that most hiking boots have.
Doubt it. Like tire compounds some rubber formulas are more hydrophilic than others and as they age and harden, lose their grip regardless of where they start out. Sounds like time to relegate these to domestic use and go boot shopping.
I've found the Continental rubber used in some Adidas hiking boots to have really good wet grip, something I value.
If there is an event near you, you can bring in your boots and perhaps get them resoled with a better grip. They call it resoling, but what they do is grind down the bottom of the sole and glue on a new grip. It comes out pretty nice and they only requested a $20 donation to a local charity. I can probably figure out how to link some photo's of the finished product if you are interested.